Children eat sugar cane and play in Brazil's Rio Negro, whose water level fell recently to reveal stone age etchings. Photograph: Ricardo Moraes/Reuters

A series of ancient underwater etchings has been uncovered near the jungle city of Manaus, following a drought in the Brazilian Amazon.

The previously submerged images – engraved on rocks and possibly up to 7,000 years old – were reportedly discovered by a fisherman after the Rio Negro, a tributary of the Amazon river, fell to its lowest level in more than 100 years last month.

Though water levels are now rising again, partly covering the apparently stone age etchings, local researchers photographed them before they began to disappear under the river's dark waters.

Archaeologists who have studied the photographs believe the art – which features images of faces and snakes – is another indication that thousands of years ago the Amazon was already home to large civilisations.

Eduardo Neves, president of the Brazilian Society of Archaeology and a leading Amazon scholar, said the etchings appeared to have been made between 3,000 and 7,000 years ago when water levels in the region were lower. The etchings were "further, undeniable evidence" that the region had been occupied by a significant number of ancient settlements and people.

"There has always been this idea that the Amazon was empty. The truth is that this hypothesis is not correct. In many parts of the Amazon we now have proof of settlements," he said, adding that the discovery was of great scientific importance.

Recent years have seen a growing number of archaeologists studying the Amazon, revising previous theories that the rainforest was too inhospitable to host a major civilisation.

"The conventional account of the Amazon basin is that it was inhabited by very small, often nomadic indigenous communities," said archaeologist Manuel Arroyo-Kalin, a research associate at University College London and Durham University familiar with the Manaus region.There was growing proof of "incredible pottery, large villages [and] roads going from one place to another" which "for a century or two" had been discarded by scholars.

With soy farmers, loggers and urban settlements advancing, cataloguing and preserving ancient Amazon sites had become a race against time.

"In the city of Manaus the amount of archaeology that has been destroyed is impressive," Arroyo-Kalin said.

Archaeologists are particularly concerned about the imminent inauguration of a 2.2-mile bridge across the Rio Negro connecting Manaus with Iranduba. The area is home to numerous archaeological sites, where ancient ceramics and burial urns have been found. "The bridge … will probably alter quite dramatically life on the other side of the Rio Negro … because [it] will put pressure on the land with urbanisation, and river fronts tend to be loaded with archaeological remains," Arroyo-Kalin said. "By changing the dynamic of how the region is being used … you will certainly start damaging archaeology."

Neves said he hoped the latest find would boost efforts to preserve the rainforest and its ancient secrets.